1. Field of the Invention
The present invention relates to a timer apparatus and, more particularly, to a timer apparatus in which a plurality of timer service requests are simultaneously processed by single timer means.
2. Description of the Prior Art
The timer function is indispensable to the time control of every apparatus. For example, the timer function is indispensable to control the excitation of each phase of a pulse motor; to monitor the timing of a communication protocol in on-line communication; or to control a paper feed sequence, exposure development sequence or the like of a copying machine. At the present time almost all of those apparatuses are controlled by way of a one-chip CPU and one or two timer means are ordinarily included in this one-chip CPU. However, in actuality the timer means included in the CPU is fairly insufficient because use of the time function is very frequently requested. Therefore, these requests are met be effecting the timer means as if it has pseudo-timer multifunctions.
FIGS. 1A and 1B show conventional methods of realizing pseudo-timer multifunctions. FIG. 1A is a diagram showing the method whereby a single timer means is used in a time-sharing manner. For example, there is a certain apparatus including control units 2 to 5 each having a special function and these units commonly use a timer 1 to perform the time control. Such a time control method is popular. According to the time-sharing method, for instance, when the control unit 2 outputs a trigger signal TG to the timer 1, the timer 1 is used only for a control unit 2 and the other control units 3 to 5 cannot use the timer 1 until a time-out signal TO is outputted to the control unit 2. Consequently, this constitution can be actually used only for the control of a limited purpose and it needs complicated management and control such as determination of the control unit priority when the timer is used, and the like.
FIG. 1B is a diagram showing the case where the timer function is seemingly distributed and each control unit has the pseudo-timer function. Clock means 6 may be an oscillator and, in many cases, is constituted by a timer service process to generate a clock signal at a regular interval (e.g., 5 msec) by using a timer including a CPU therein. In this case, each of control units 7 to 10 has timer means (for example, a counter and a comparator) and allows an independent set value to be held in the counter in response to generation of a request for the timer service. Therefore, whenever a service request is generated from the clock means, the content of each counter is decreased by "1", so that each of the control units 7 to 10 is allowed to make its own timer means operative simultaneously. This constitution, therefore, has various uses. However, when the clock means 6 is in the timer service of the CPU, the service interval inevitably becomes long, so that a high-speed and high-accuracy timer function cannot be expected. On one hand, if such a constitution is realized by hardware, each of the control units 7 to 10 must be equipped with the corresponding timer hardware, resulting in an increase in size cost of the apparatus.